docs: def fetch_credential(self, credential=None, profile=None):
          """Fetch credential from credentials file.
  
          Args:
              credential (str): Credential to fetch.
              profile (str): Credentials profile. Defaults to ``'default'``.
  
          Returns:
              str, None: Fetched credential or ``None``.
  
          """
          q = self.db.get(self.query.profile == profile)
          if q is not None:
              return q.get(credential)